Fabrication of semiconductors, flat panel displays, and photovoltaics (PV) or solar cells require multiple processes, such as etching, chemical vapor deposition, sputtering and cleaning, all of which are performed on various substrates to form the desired device or product. Each of these processes may be performed using a single and distinct processing tool or module that performs a single fabrication process. Since multiple fabrication processes must be performed, substrates must be transferred from one processing tool to the next, which exposes the substrates to breakage and contamination. Further, transferring substrates between different processing tools increases the overall processing time and cost of fabrication.
A variety of process architectures are used in the industry. Traditional inline processing tools, which arrange processing tools linearly and move substrates sequentially from one processing tool to the next processing tool, are known to be inefficient, particularly when each processing tool requires different processing time as is commonly the case. For example, bottlenecks are common when substrates processed by a faster tool have to wait for their respective turn to be processed by a slower, downstream process tool.
Consequently, system architectures have been developed that provide multiple processing tools that can perform multiple fabrication processes. One commonly used example of a multiple processing tool is a cluster tool. The cluster tool employs multiple process chamber units arranged in a circular fashion typically connected to a single, large immobilized vacuum transfer chamber with one vacuum transfer robot to transfer substrates between the process chambers via multiple load lock chambers. Since substrates are transferred within a single tool for different fabrication processes, the potential for contamination is reduced. In addition, the substrates can be more quickly transferred between process chamber units, which reduces the overall processing time.
Traditional cluster tools however suffer several significant limitations. First there is a practical limit in the number of fabrication tools that may form the cluster. In order to add fabrication tools to the cluster, the transfer chamber size needs to increase to provide sufficient area to transport substrates from the transfer chamber to process chambers. This requires a long-reach transfer robot. Furthermore, adding a new tool to the cluster may require a whole new cluster tool if the capacity of the existing cluster tool is not sufficient to accommodate the new tool. Thus, the system is not easily expanded.
Second, the large immobile vacuum transfer chamber is of complex mechanical design and is not easily adapted to accommodate large substrates. For example, large glass or silicon substrates for photovoltaic or flat panel applications require a large rotating radius to turn the correspondingly large vacuum transfer chamber, and requires a large vacuum pump and expensive robot components that are rigid enough to perform such long stroke of travel.
Additionally, certain photovoltaic and semiconductor products involve processing steps of varied duration, causing significant bottlenecks in the processing line. For example, photovoltaic cells require deposition of multiple thin film layers of various thickness. Deposition of an intrinsic layer (“I-layer”), negative or n-doped layer (“N-layer”), and positive or p-doped layer (“P-layer”) often require significantly different deposition time to achieve the desired thickness. When deposition of a layer requiring short deposition time is followed by deposition of a layer requiring a long deposition time, the second layer creates a bottleneck and limits the throughput especially in a sequential or inline manufacturing process. The fabrication of multi-junction photovoltaic cells further magnifies the problem.
Accordingly, further improvements are needed.